The fast-food chain is facing a false-advertising lawsuit that alleges its seasoned beef is mostly fillers. But several experts say the additives are quite common in processed foods.

Taco Bell fans have spent the last week wondering what's really in their meals after a lawsuit was filed alleging that the popular fast-food chain's meat contains a whole lot of mystery.

Some consumers cringed at the term "taco meat filling," which is how the lawsuit says Taco Bell should advertise its seasoned beef. It alleges that the product contains mostly substances other than beef.

Taco Bell Corp., a Yum Brands Inc. subsidiary based in Irvine, has fired back, refuting the lawsuit's allegations and defending its menu ingredients.

As it turns out, the lawsuit's allegations — and the stomach-churning terminology — hinge partly on regulatory language that is meant to be used by manufacturers for labeling purposes, not restaurants. There also aren't any hard rules that define what a company or restaurant can advertise as meat.

"Obviously you know it's not 100% organic food," said Taco Bell customer Bethany Weis, 23, of Chicago. "I know it's not good for me. I still like it."

In striking back against the suit, Taco Bell states that its beef recipe is 88% beef and 12% seasonings, spices, water and other ingredients. Some of those "other ingredients" aren't things you are likely to add to your own beef for family taco night, but several experts say the additives are quite common in processed foods.

Alabama attorney W. Daniel "Dee" Miles III started the beef brouhaha after filing a false-advertising suit that contends Taco Bell mislabels its products by telling consumers they are eating "beef" or "seasoned ground beef" despite having the product labeled internally as "taco meat filling."

That jargony term comes from the U.S. Department of Agriculture, which has a 202-page labeling and policy book that is designed to help manufacturers prepare product labels that are truthful and not misleading.

According to the USDA, which regulates the nation's meat supply, "taco meat filling" is required to contain at least 40% fresh meat and must be labeled with the product name, including the word "filling."

But that requirement applies to raw meat sold by manufacturers. The USDA doesn't regulate what companies such as restaurants can describe to their customers in advertisements as "beef," "chicken" or "meat," USDA press officer Neil Gaffney said.

The Federal Trade Commission is the agency that regulates whether advertising is deceptive. The FTC has no specific rules that define what can be advertised as meat or beef, said Betsy Lordan, an FTC spokeswoman.

In the lawsuit, Miles includes what appears to be a label from Taco Bell's raw meat mixture, which reads "taco meat filling."

Miles said in an interview that he had the company's meaty mix tested and found that, overall, 15% was protein. Miles wouldn't turn over his laboratory reports to Tribune Newspapers, and after the story became a nationwide phenomenon, he stopped answering questions about it.

But that low percentage might not be as surprising as it sounds. Ground beef alone is more than half composed of naturally occurring water, according to the USDA. And it's common for food manufacturers to add seasonings and other ingredients to food, said Susan Brewer, a professor in the department of food science and human nutrition at the University of Illinois at Urbana-Champaign.

It's completely expected that meat would contain about 12% protein, with most of the rest being water and fat, Brewer said. "Protein is not the major component," she said.

And if you've ever had a Taco Bell taco, or any taco for that matter, you know the brown, spicy meat mixture contains more than just beef.

In its public-relations rebuttal to the lawsuit, Taco Bell said its seasoned beef includes "ingredients you'd find in your home or in the supermarket aisle." It goes on to name ingredients that sound reasonable: salt, chili pepper, onion powder, tomato powder, sugar, garlic powder, even cocoa powder.

But there are also some seasoned beef ingredients Taco Bell left out of its national ad campaign last week to refute allegations in the lawsuit — ingredients you might have a tough time finding in your home pantry or grocery store.

Soy lecithin, for example. It's a byproduct of soybean processing that is used as an emulsifier. That means it helps blend and bind substances that would otherwise separate, like oil and water.

And then there's autolyzed yeast extract. Made by breaking down yeast cells with salt, it's a flavor-enhancing additive similar to monosodium glutamate, without the side effects of MSG some people experience. It gives foods a full, savory, beeflike taste, Brewer said.

Maltodextrin is derived from starches, usually corn in the U.S. It can be used as a sweetener and a thickener.